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ignition,apparatus for igniting a combustible mixture. The German engineer Nikolaus A. Otto, in his first gas engine, used flame ignition; another method was heating a metal tube to incandescence. Ignition systems in modern automobiles use an electric spark to ignite the compressed mixture of air and gasoline in the cylinders.
Battery Ignition Systems
A battery ignition system has a 6- or 12-volt battery charged by an engine-driven generator to supply electricity, an ignition coil to increase the voltage, a device to interrupt current from the coil, a distributor to direct current to the correct cylinder, and a spark plug projecting into each cylinder. Current goes from the battery through the primary winding of the coil, through the interrupting device, and back to the battery.
Interrupting the Current
In older automobiles, the interruption of the primary current was created by "breaker points," a switch with tungsten contacts to retard erosion. Driven at half engine speed, a breaker cam, a rotating object with a lobed surface (one lobe for each cylinder), opened and closed the points. When the breaker points were closed, current flowed through the primary winding of the ignition coil. In electronic ignition systems, introduced in the early 1960s, the interrupting device is a reluctor, a magnetic pulse distributor that produces timed electric signals that are amplified to control the current to the primary winding of the ignition coil. Such systems generally reduce ignition maintenance and increase engine efficiency.
The Ignition Coil and Distributor
The primary winding consists of wire coiled around an iron core. Over this is a secondary winding of many more turns of finer wire attached to the distributor. Current flowing through the primary winding creates a magnetic field. When the breaker cam opens the breaker points or the reluctor delivers its signal, the circuit is broken and current stops. The magnetic field collapses, inducing in the secondary winding a much higher voltage that is led to the distributor. Inside the distributor a moving finger rotates at half engine speed. As it rotates it touches contacts, each of which runs to a different cylinder. Rotation is timed so that when the finger is touching the contact for a particular cylinder, a high voltage has just been induced in the secondary winding of the ignition coil and the piston has almost reached the top of the compression stroke. Thus a high voltage is impressed across the spark plug gap.
The Spark Plug
The spark plug consists of a center electrode imbedded in insulating ceramic. Around the outside is a threaded metal shell that screws into a hole in the top of the cylinder. A ground electrode extends from the shell over the end of the center electrode. Between the two electrodes there is a small gap of .015–.040 in. (.038–.102 cm). At about 8,000 volts a spark jumps the gap and ignites the air-gasoline mixture. A centrifugal advance makes the spark fire earlier at high engine speeds; a vacuum advance makes it fire earlier at small throttle openings above idle.
Magneto and Diesel Ignition Systems
A magneto ignition system is essentially the same as a battery system except that a permanent magnet generator supplies current directly. Where compactness is an advantage or where there are no other accessories that require a battery, a magneto system may be preferred. Aircraft, motorcycles, and farm equipment often have magnetos.
In a diesel engine the fuel ignites as soon as it is injected into the hot, highly compressed air in the cylinder. Diesel engines frequently utilize an electric heating element, called a glow plug, inside the cylinder to preheat the air to facilitate starting and running until the engine has reached its operating temperature.
in internal-combustion engines the forced firing of the working mixture in the combustion chamber of an engine.
Two ignition systems have become widespread: the battery type and the magneto type. In the battery type, low-voltage electric current (6 or 12 volts) passes from a storage battery through an ignition switch into the primary winding of an induction coil. High-voltage current (12,000 to 15,000 volts) induced in the secondary winding of the coil is conducted to the rotor of the ignition distributor. Current from the rotor goes to a contact, from which it is delivered to the spark plug of the cylinder in which the ignition of the working mixture is to occur. The operation of the internal-combustion engine depends on how the ignition is timed. For effective operation it is necessary for the spark between the electrodes of the spark plug to occur before the piston reaches top dead center, that is, for ignition to occur with a slight amount of advance. The ignition advance is expressed in degrees of the angle of rotation of the crankshaft and depends on the frequency of rotation, the load on the motor, and the type of fuel used. Optimum angles of ignition advance are determined automatically while the motor is in operation (by an automatic centrifugal-vacuum device). Battery type ignition has the advantages of simple design, low cost, and reliable starting; it is used chiefly in motor vehicles. Ignition from a magneto ensures dependable operation, does not require a storage battery, and is used chiefly on tractors, motorcycles and special motor vehicles.
One disadvantage of battery ignition is that the voltage in the secondary circuit drops as the rotation of the crankshaft increases. This prevents increases in internal-combustion engine speed and compression and limits use of lean fuel mixtures. It also leads to rapid wear of the points of the distributor, thus decreasing the reliability of the ignition system. To overcome these problems, transistor ignition systems are coming into use. They have a much higher spark discharge voltage (30,000 volts), which facilitates starting and assures more complete combustion, while some-what reducing the toxicity of the exhaust gases.
REFERENCESGalkin, lu. M. Elektrooborudovanie avtomobilei i traktorov, 2nd ed. Moscow, 1967.
Nabokikh, V. A., and V. I. Cheplanov. Osnovnye tendentsii razvitiia elektronnykh sistem zazhiganiia avtomobil nykh dvigatelei. Moscow, 1967.
B. A. KUROV